Back in the Paolo's Plunge thread, someone recently made a statement to the effect that the idea of an early wet, warm Mars has gone from a proposition to a belief. The statement made it clear that this was a bad thing -- that the concept of an early wet, warm Mars is keeping us from seeing how the planet's histopry has actually played out.

I'm responding to that statement here since I don't want to continue to hijack the other thread. But I feel the following two points must be made:

1) Mars was once warm enough (and had a thick enough atmosphere) to support flowing liquid water on its surface.

2) Mars was once wet enough for that liquid water to form well-developed river drainage systems and for enormous floods to scour thousands of square kilometers of its surface.

Those two statements aren't theoretical. Observed landforms verify them with primary, empirical evidence of river channels and catastrophic flood plains.

Those are statements of fact, not belief. My own feeling is that we must proceed from that point and not continually try and postulate a Martian history which cannot account for these proven facts.

Also, to the comment made several times that the LHB was responsible for the stripping of Mars' atmosphere, I must point out that several reputable studies have shown that the interaction between the solar wind and Mars' upper atmosphere is sufficient to have reduced an atmosphere as dense as Earth's to what we see today over the course of three billion years. And that neither Venus nor Earth seem to have had their atmospheres stripped during the LHB.

Just a few points I felt needed to be made at this juncture.

-the other Doug

--------------------

“The trouble ain't that there is too many fools, but that the lightning ain't distributed right.” -Mark Twain

Good points. I'm not sure that we can state definitely what happened to planetary atmospheres during the LHB but given the smaller size of Mars compared to Earth and Venus the chances of a given large impactor creating a plume exceeding the escape velocity would seem to be much higher. While the LHB would have given the Martian atmosphere a real shakeup, polar deposition and weathering would also have contributed and since Mars lost it's prime magnetic field (possibly as a consequence of the LHB) the minor residual fields would have provided little protection from the effect of the solar wind and the remaining atmosphere would have been steadily degraded despite any recharging from volcanic activity. But the loss of the atmosphere remaining after the LHB would probably have taken some considerable time which fits with the apparent gradual decrease in erosion over time.

Deposition by impact surge requires an atmosphere else all ejecta is ballistically emplaced. So no matter which camp you may follow in the endless saga on the genesis of Meridiani and Home Plate, both require a warmer wetter Mars with an atmosphere significantly thicker than the current vacuum.

I had this table handy. Based on estimated erosion rates, it seems that the early [Noachian] erosion rate is comparable to erosion rates in Earth's dry deserts. Later erosion rates (presumably eolian) drop significantly.

I somehow missed this extemely interesting, short-lived discussion a month ago (because I was in Houston giving a talk on liquid water), but let me comment that most of the Noachian Mars drainage features can and have (by others) been explained as a result of the LHB itself, assuming that Mars was at least icy. They coincided in time - the Noachian basically ended when the LHB did. Large impacts would vaporize sufficient steam and other volatiles to create a very short-lived greenhouse; steam condensation could then create drainage networks accompanying catastropic floods and ice-covered lakes with deltas and even clay minerals (water would convert impact glasses to clay minerals extremely rapidly). Then dry and cold until the next impact. Smaller impacts towards the tail end of the LHB would have somewhat less effect on climate, but should still deposit sediments (cross-bedded surge and fallout deposits), because each impact into ice generates its own vapor. (Example: young rampart craters.) Late, small impacts especially might condense frost or snow rather than liquid. Volcanism would play a secondary, local role by comparison.

Blaming up to 99% of the early atmospheric loss on the LHB (with slow, steady losses afterwards owing to solar effects) is regarded as mainstream among Mars atmospheric modelers (Catling et al.). Early Mars may have been ephemerally warm and wet planetwide, but probably only immediately after a major impact episode. And each major impact brought it that much closer (doomed it to) a dry and cold future (at least on the surface). Therefore all the old craters are still there for us to look at, at least in the highlands. And even old craters on the Moon are degraded, although in somewhat different ways (e.g., no terrain softening from ground ice).

BTW, I agree with Aussie regarding his contrast of Mars with the much larger Earth and Venus, but disagree with his contention that a dense, permanent martian atmosphere must have been present to generate an impact surge, such as at possibly Meridiani or Home Plate. A volatile-rich target region should suffice (i.e., the atmosphere is temporarily generated by the impact; see above), and even the thin present atmosphere of Mars seems sufficient to move dust and sand.

This not to say that a long-lived warm, wet, densely-atmosphered, Earth-like Mars wasn't initially present. It simply isn't required by any data I've seen to date, runs contrary to simple observations like visible surface cratering, planet size, and distance from the sun, and most of the newer scientific data seem to be pointing the other way.

...What observational evidence would allow differentiation of periods of Scenario A or periods of Scenario B?(BTW, Mars not the only body for which this question seems relevant)- Mike

Mike - Excellent question. I'll try to answer it, but doubt you'll like my answer. First, if Mars was completely slagged during the LHB (i.e., the surface was remelted), as probably happened to Earth, then the question becomes unanswerable (and probably irrelevant), because virtually all earlier history would be completely erased (except perhaps as zoning in zircons or other difficult-to-melt minerals). Clearly, after the LHB had tailed off, the surface seems to have remained largely dry and cold, or else the LHB cratering record should have been erased by weather.

Second, if the surface was not completely slagged by the LHB, then the earlier Earth-like warm, wet period should have left a record in the form of abundant clay minerals in shales, and possibly carbonate sediments (especially if life was around). But I'm not sure how you'd definitively tell those few sedimentary remnants apart from sediments generated during a temporary LHB greenhouse, without radiometic age dating of interbedded lavas (i.e., without Mars sample return). The apparent restriction of clay minerals to the oldest, most heavily cratered terrain, and abundance of fresh igneous minerals (olivine, pyroxene, and plagioclase) in younger sediments seems to provide independent evidence that the surface remained largely dry and cold afterwards. The abundance of soluble salts and acid salts on the surface also implies cold and dry (liquid water would dissolve or destabilize them). In other words, geomorphology and mineralogy seem to agree about cold and dry afterwards (anomalous Meridiani claims notwithstanding).

So we seem to know that post-LHB Mars was largely dry and cold on the surface (at least at the limits of orbital observation and detection), but the record of anything earlier seems ambiguous (in terms of distinguishing your two scenarios). As our orbital resolution increases, more landers arrive, and Mars sample return becomes a reality, hopefully we'll learn more.

BTW, from what I read, post-Noachian outflow channels seem to involve groundwater (presumably brine) abruptly escaping during collapse of chaotic terrain at their source. I've been discussing long term surface surface climate and weathering, not local subsurface conditions that might have led to catastrophic escape of brine.

Patteroast, regarding Ceres I suspect (without recalling any relevant modeling) that a temporary greenhouse would only work on planets above a certain size (but still too small to hold a dense, permanent atmosphere). Mars qualifies, but Ceres might not.

Sounds like another good argument for a sample return mission. (And extra care to hopefully finding just the right layering for deciphering the early martian record).

[The other body I was thinking of was Titan. Not sure if the earlier presumably wetter methane abundance was steady (Scenario A) or more post-impact espisodic (Scenario B). Still, chemical analysis of surface materials might be a way to tell, assuming appropriate layering could be found, dissected, and analyzed in a future mission (trapped isotopic gas ratios in ice grains being the timing marker?). Sample return, anyone?]

Clearly, after the LHB had tailed off, the surface seems to have remained largely dry and cold, or else the LHB cratering record should have been erased by weather.

Well... I completely agree with you (especially with that proviso "largely"), as far as Mars' southern hemisphere is concerned. But north of the dichotomy line (which isn't exactly equatorial), very few remnants of the LHB are seen to survive. That land *has* been heavily resurfaced, somehow, since the LHB.

So -- *half* of Mars fits your description above. Half doesn't.

Makes it deucedly difficult to come up with generalized descriptions of the planet's past, doesn't it?

-the other Doug

--------------------

“The trouble ain't that there is too many fools, but that the lightning ain't distributed right.” -Mark Twain

When the LHB petered out Mars would probably have been left with a remnant of its initial atmosphere. But Earth would not have been particularly better off. Earth's atmosphere was recharged from volcanic activity and given the clear indications of significant volcanic activity on Mars, the post LHB build up of a reasonable atmosphere consisting of greenhouse gas would seem an acceptable concept. Mars was just not big enough to hold the atmosphere but while it lasted it would probably have enabled a warmer, dynamic, erosional environment.

The thing I find interesting is that the composition of the major gasses in the remaining Martian atmosphere is similar to that of Venus. Over 95% carbon dioxide and some 3% nitrogen. So Earth is the anomaly (if that expression is allowed with only 3 planets in the goldilocks zone) and it would seem that the key influence for Earth was abundant water and biological processes which over a long period removed carbon dioxide, injecting waste products like oxygen and some nitrogen from denitrification in its place. Following this rather tenuous logic the atmospheric gas percentages in the Martian atmosphere seem to point to a lack of any biological influence in the past.

How long would it take earth's atmosphere to change if all life here disappeared tomorrow? And how large does the biosphere have to be to maintain atmosphere in its current state? The compositions of mars and venus atmospheres may well point to no significant biological activity today, but I don't agree that it would rule it out in the deep past. The only other world we know of with a thick nitrogen atmosphere is titan, and the common link there is organic chemistry rather than life (or at least life as we know it).

So Earth is the anomaly (if that expression is allowed with only 3 planets in the goldilocks zone) and it would seem that the key influence for Earth was abundant water and biological processes which over a long period removed carbon dioxide, injecting waste products like oxygen and some nitrogen from denitrification in its place.

Earth is also the only planet with smoothly running plate tectonics (probably lubed by water).

The exchange with the ocean and precipitation of carbonate salts probably did a great job of sequestering a lot of Earth's CO2 on Earth. Then stuffing precipitated carbonates in a trench due to tectonic action is a really great way to take CO2 out of the game for a nice long while.

I'm not sure of the relative removal rates of CO2 due to geological, oceanic, and biological processes. But I'll wager that all these helped make Earth a really nice place to live. (Most of these processes went away during the putative Snowball Earth phase, leading to the buildup of CO2 which eventually caused the Big Melt)

(I'd speculate that if plate tectonics magically stopped during the Big Melt (or any other time early in Earth's history), our atmosphere would eventually resemble Venus'.)

if Mars was completely slagged during the LHB (i.e., the surface was remelted), as probably happened to Earth

In that case would pre-LHB zircon crystals really have survived intact? And it seems that the Acasta formation in Canada is older than at least the later part of the LHB. If the surface was completely remelted I would think that all the radiologic ages would have been reset.

IMAGE COPYRIGHT
Images posted on UnmannedSpaceflight.com may be copyrighted.
Do not reproduce without permission. Read
here for further information on space images and copyright.

OPINIONS AND MODERATION
Opinions expressed on UnmannedSpaceflight.com are those of the
individual posters and do not necessarily reflect the opinions
of UnmannedSpaceflight.com or The Planetary Society. The all-volunteer
UnmannedSpaceflight.com moderation team is wholly independent
of The Planetary Society. The Planetary Society has no influence
over decisions made by the UnmannedSpaceflight.com moderators.

SUPPORT THE FORUM
Unmannedspaceflight.com is a project of the Planetary Society
and is funded by donations from visitors and members. Help keep
this forum up and running by contributing
here.